Nanocrystalline materials are a new class of disordered solids which have a large volume fraction (50% or more of the atoms) of defect cores and strained crystal lattice regions. The physical reason for the reduced density and the non-lattice spacing between the atoms in the boundary cores is the misfit between the crystal lattice of different orientation along common interfaces. The nanocrystalline system preserves in the crystals a structure of low energy at the expense of the boundary regions which are regions at which all of the misfit is concentrated so that a structure far away from equilibrium is formed (Gleiter, Nanocrystalline Materials, Prog. in Matls Science, Vol 33, pp 223-315, 1989). A structure of similar heterogeneity is not formed in thermally induced disordered solids such as glasses. Nanocrystalline materials typically have a high density (10.sup.19 per cm.sup.3) of grain interface boundaries. In order to achieve such a high density, a crystal of less than about 10 0 nm diameter is required. Over the past few years, great efforts to make smaller and smaller nanocrystals, down to about 10 nm have been made. It would appear, however, that the properties of even smaller nanocrystals (less than 10 nm) offer significant advantages over larger nanocrystals, particularly in the area of hardness, magnetic behavior hydrogen storage, and wear resistance.
Nanocrystalline materials, which are also known as ultrafine grained materials, nanophase materials or nanometer-sized crystalline materials, can be prepared in several ways such as by sputtering, laser ablation, inert gas condensation, oven evaporation, spray conversion pyrolysis, flame hydrolysis, high speed deposition, high energy milling, sol gel deposition, and electrodeposition. Each of these methods has its special advantages and disadvantages and not all methods are suitable for all types of nanocrystalline materials. It is becoming apparent, however, that electrodeposition is the method of choice for many materials. The major advantages of electrodeposition include (a) the large number of pure metals, alloys and composites which can be electroplated with grain sizes in the nanocrystalline range, (b) the low initial capital investment necessary and (c) the large body of knowledge that already exists in the areas of electroplating, electrowinning and electroforming.
Using electrodepositing techniques, nanocrystalline electrodeposites of nickel and other metals and alloys have been produced over the years with ever smaller diameters down to the 10-20 nm range. Heretofore, it has not been possible to get sizes below about 10 nm diameter. Small crystal sizes increase the proportions of triple junctions in the material. It is known that room temperature hardness increases with decreasing grain size in accordance with the known Hall-Petch phenomenon. However, it has now been determined that as the number of triple junctions in the material increases, at about 20 nm down, there is a deviation from normal Hall-Petch behavior and hardness does not continue to increase as the grain size falls below a critical value. Indeed, it has now been shown that in pure nickel nanocrystalline materials the hardness reaches a peak in the 8-10 nm range. Other materials even show a decrease in hardness as the grain size decreases below about 10 nm.
Nanocrystalline materials have improved magnetic properties compared to amorphous and conventional polycrystalline materials. Of particular importance is the saturation magnetization, which should be as high as possible regardless of grain size. However, previous studies on gas-condensed nanocrystalline nickel (Gong et al, J. Appl. Phys 69, 5119, (1991)) reported decreasing saturation magnetization with decreasing grain size. It would appear, however, that this phenomenon is associated with the method of production as electroplated nanocrystalline nickel in accordance with the present invention shows little change in saturation magnetization.